The purpose of a catalytic converter is to enhance a chemical reaction, for example, oxidation. When used with an internal combustion engine or a gas turbine, its purpose is to convert pollutant materials in the exhaust, e.g., carbon monoxide, unburned hydrocarbons, nitrogen oxide, etc., to carbon dixoide, nitrogen, and water. Conventional catalytic converters utilize a ceramic honeycomb monolith having square or triangular straight-through openings or cells with catalyst deposited on the walls of the cells; catalyst coated refractory metal oxide beads or pellets, e.g., alumina beads; or a corrugated thin metal foil honeycomb monolith, e.g., a ferritic stainless steel foil or a nickel alloy foil, having a catalyst carried on or supported on the surface. The catalyst is normally a noble metal, e.g., platinum, palladium, rhodium, ruthenium, or a mixture of two or more of such metals. Zeolite coating may also be used for adsorption and desorption of the pollutants to aid in catalytic activity. The catalyst catalyzes a chemical reaction, mainly oxidation, whereby the pollutant material is converted to a harmless by-product which then passes through the exhaust system to the atmosphere.
However, conversion to such harmless by-products is not efficient initially when the exhaust gases are relatively cold, e.g., at cold start. To be effective at a high conversion rate, the catalyst and the surface of the converter with which the gases come in contact must be at or above a minimum temperature, e.g., 390 F. for carbon monoxide, 570 F. for volatile organic compounds (VOC) and 100 F. for methane or natural gas. Otherwise, conversion to harmless by-products is poor and cold start pollution of the atmosphere is high. It is estimated that as much as 80% of the atmospheric pollution from internal combustion engines occurs within the first 2 minutes of operation. Once the exhaust system has reached its normal operating temperature, the catalytic converter is optimally effective. Hence, it is necessary for the relatively cold exhaust gases to make contact with a hot catalyst so as to effect satisfactory conversion. Compression ignited engines, spark ignited engines, and reactors in gas turbines have this need.
To achieve initial heating of the catalyst at or prior to engine start-up, there is conveniently provided an electrically heatable catalytic converter, preferably one formed of a thin metal honeycomb monolith, which may be spaced flat thin metal strips, straight corrugated thin metal strips, pattern corrugated thin metal strips, (e.g., herringbone or chevron corrugated) or variable pitch corrugated thin metal strips (see U.S. Pat. No. 4,810,588 dated Mar. 7, 1989 to Bullock et al), or a combination thereof. The monolith is connected to a voltage source, e.g., a 12 volt to 108 volt or higher DC power supply, preferably at the time of engine start-up and afterwards to elevate and maintain the catalyst to at least 650 F. plus or minus 30 F. Alternatively, power may be supplied for a few seconds prior to engine start-up.
Catalytic converters containing a corrugated thin metal (stainless steel) monolith have been known since at least the early 19070's. See Kitzner U.S. Pat. Nos. 3,768,982 and 3,770,389 each dated Oct. 30, 1973. More recently, corrugated thin metal monoliths have been disclosed in U.S. Pat. No. 4,711,009 to Cornelison et al dated Dec. 8, 1987; U.S. Pat. No. 4,381,590 to Nonnenmann et al dated May 3, 1983; U.S. Pat. No. 5,070,694 to Whittenberger dated Dec. 10, 1991; and International PCT Publication Numbers WO 89/10470 (EP 412,086) and WO 89/10471 (EP 412,103) each filed Nov. 2, 1989, claiming a priority date of Apr. 25, 1988. The above two PCT Publications disclose methods and apparatus for increasing the internal resistance by placing a group of spaced discs in series or insulating intermediate layers. Another International PCT Publication Number is WO 90/12951 published Apr. 9, 1990 and claiming a priority date of Apr. 21, 1989 which seeks to improve axial strength by form locking layers of insulated plates. Another reference which seeks to improve axial strength is U.S. Pat. No. 5,005,275 dated Oct. 8, 1991 to Kannianen et al. However, a common problem with such prior devices has been their inability to survive severe automotive durability tests which are known as the Hot Shake Test and the Hot Cycling Test.
The Hot Shake Test involves oscillating (100 to 200 Hertz and 28-60 G inertial loading) the device in a vertical attitude at high temperature (between 800 and 950 C.; 1472 to 1742 F., respectively) with exhaust gas from a running internal combustion engine being simultaneously passed through the device. If the device telescopes, or displays separation or folding over of the leading or upstream edges of the foil leaves up to a predetermined time, e.g., to 5 to 200 hours, the device is said to fail the test. Usually, a device that lasts 5 hours will last 200 hours. Five hours is equivalent to 1.8 million cycles at 100 Hertz.
The Hot Cycling Test is conducted with exhaust flowing through at a temperature of 800 to 950 C. (1472 to 1742 F.) and cycled to 120 to 150 C. once every 15 to 20 minutes, for 300 hours. Telescoping or separation of the leading edges of the foil strips is considered a failure.
The Hot Shake Test and the Hot Cycling Test, hereinafter called "Hot Tests," have proved very difficult to survive, and many efforts to provide a successful device have been either too costly or ineffective for a variety of reasons.
Previously stated samples of electrically heatable catalytic converters (EHC) in automotive service and comprised entirely of heater strips in electrical parallel did not have adequate endurance in the Hot Tests nor did they have sufficiently high resistance to fulfill the need for lower power ratings. In repeated efforts to arrive at a suitable design using purely parallel circuit construction, samples were made with a wide range of parameters, including a length-to-diameter aspect ratio of from 0.2 to 1.5, cell densities of from 100 to 500 cells per square inch, individual strip heaters as long as 20 inches, and parallel circuits limited to as few as 2 to 4 heater strips.
Devices made according to these parameters have generally proved unsatisfactory in the Hot Tests because (a) the terminal resistance was too low and, therefore, the devices drew too much power, (b) the relatively high voltage differential between laminations associated with small numbers of parallel heater strips caused some interlaminar arcing, and (c) Hot Tests could not be passed consistently. Resistance that is too low causes one or more of the following problems: (a) the battery, cabling and switching apparatus becomes unacceptably large and expensive; (b) the EHC has to be made with longer heater strips which have a tendency to fail the Hot Tests.
Copending and commonly owned U.S. patent application Ser. No. 826,488 filed Jan. 27, 1992 discloses a generally circular electrically heatable catalytic converter which is capable of surviving the Hot Tests. The present invention is an improvement on this device in that it provides in one embodiment, a device that is oval in cross-section and is capable of surviving the Hot Tests. The oval cross-section enables better ground clearance and better fit-up with existing oval shaped converters. Moreover, these devices are capable of being made in smaller axial dimension, especially for use with smaller displacement engines, e.g., from about 1.5 to 3.0 liters and still meet EPA requirements as of this date on emissions.
In the following description, reference will be made to "ferritic" stainless steel. A suitable formulation for ferritic stainless steel alloy is described in U.S. Pat. No. 4,414,023 to Aggen dated Nov. 6, 1983. A specific ferritic stainless steel useful herein contains 20% chromium, 5% aluminum, and from 0.002% to 0.05% of at least one rare earth metal selected from cerium, lanthanum, neodymium, yttrium, praseodymium, or a mixture of two or more of such rare earth metals, balance iron and trace steel making impurities. Another metal especially useful herein is 99.5% nickel. Still another nickeliferous alloy useful herein is identified as Haynes 214 alloy which, like ferritic stainless steel alloy above described, is commercially available. This alloy is described in U.S. Pat. No. 4,671,931 dated Jun. 9, 1987 to Herchenroeder et al. The alloy is characterized by high resistance to oxidation. A specific example contains 75% nickel, 16% chromium, 4.5% aluminum, 3% iron, optionally trace amounts of one or more rare earth metals except yttrium, 0.05% carbon, and steel making impurities. Still another nickeliferous alloy useful herein is Haynes 230. This alloy contains 22% chromium, 14% tungsten, 2% molybdenum, 0.1% carbon and a trace amount of lanthanum, balance nickel. Ferritic stainless steel (commercially available as Alfa IV from Allegheny Ludlum Steel Co.) and Haynes 214 and 230 are examples of high temperature resistive, corrosion or oxidation resistant metals that are suitable for use in making heater strips for the EHC cores hereof. Suitable metals and alloys must be able to withstand temperatures of 900 C. to 1100 C. over prolonged periods.
Other high temperature resistive, oxidation resistant metals are known and may be used herein. For automotive applications, for example, the thickness of the thin metal foil strips is in the range of from 0.001" to 0.005", preferably 0.0016" to 0.002".
In the following description, reference will also be made to fibrous ceramic mat, woven ceramic fabrics, or insulation. Reference may be had to U.S. Pat. No. 3,795,524 dated Mar. 5, 1974 to Sowman and to U.S. Pat. No. 3,916,057 to Hatch dated Oct. 28, 1975 for formulations and manufacture of ceramic fiber tapes and mats useful herein. One such woven ceramic fiber material is currently available from 3-M under the registered trademark "NEXTEL" 312 Woven Tape useful for isolating groups of heater strips as described below. Ceramic fiber mat is currently available as "Interam" also from 3-M.